Sodar systems employ sound waves to detect atmospheric phenomena such as wind speed. A monostatic sodar operates by transmitting directional sound pulses and detecting reflected signals from a single apparatus; bistatic systems have separate transmitters and receivers. Phased-array monostatic sodars employ groups of acoustic transducers to emit and receive sound beams in different directions by electronic means. This is accomplished by varying the phase of transmitted signals from the individual transducers comprising the array and by varying the phase of the sampling process such that the transducers detect the signals reflected back from the atmosphere. The array itself remains physically motionless in operation. This approach is described in U.S. Pat. No. 4,558,594, the disclosure of which is incorporated herein by reference.
The phased array approach has the benefit that the directional power density of transmitted signals, and the directional sensitivity of the array to received signals, have a primary beam width which is extremely narrow compared to what is possible with a single transducer, and which can, with appropriate electronics, be oriented in a variety of directions.
Monostatic sodar systems typically use an array of transducers arranged in a rectangular grid packing arrangement such that the transducers are aligned in rows and columns, as shown in FIGS. 2, 4 and 5 of the U.S. Pat. No. 4,558,594 patent. These arrays are operated so that they emit three sequential beams, one normal to the plane of the array, and two tilted in altitude relative to the array and 90 degrees from one another in azimuth. The rectangular grid packing arrangement, with circular transducers, leaves about 27% of the array as open space, which results in non-uniformity of sound pressure across the array, leading to potential measurement errors. Also, this inherently reduces the maximum intensity of the sound pressure, which reduces the array accuracy and sensitivity. Further, the use of asymmetric sound beams results in asymmetric sensing, which causes measurement and calculation errors.
Phased arrays ideally produce controllably aimed, sharply delineated directional cones of sound when transmitting, and equivalently shaped cones of sensitivity when receiving. In reality such arrays suffer from deficiencies. For example, the cones are not perfectly delineated by a well-defined boundary; intensity gradually drops as the angle off the main beam axis increases. Also, the cones are not perfectly shaped, but deviate from perfectly circular contours of equal sound intensity centered on the main beam axis. Further, sound transmission and reception is not entirely within the intended cones, but includes intensity outside of the intended cone of both a directionally focused and a quasi-omnidirectional nature.
The amount of sound energy radiated in undesired directions can be minimized by several techniques, including optimizing the number and physical arrangement of the individual transducers. Unfortunately it is understood that the phased array approach in and of itself cannot create a perfect directional beam. Using a practical number of transducers, the array inevitably emits sound in transmit mode, and is sensitive to sound in receive mode, in a number of directions other than the desired beam direction. For example, one or more side lobe beams are generated. Such side lobes are not as strong in transmission or sensitive in reception as the main beam, but are intense enough to degrade the performance of the sodar. These side lobes are predicted by theoretical modeling, and their existence is confirmed by experimentation.
Also, the main beam intensity does not abruptly, or even monotonically, drop to a background level as the angle of measurement deviates from the beam center axis. Instead, intensity drops to a first null, then increases somewhat from this null to a higher level, then drops to a second null, and so on. This results in the main beam being surrounded by one or more annular “rings” of sound, at angles to the main beam axis. These annular rings are also intense enough to degrade the operation performance of the sodar. Like the side lobes, these annuli are also predicted by theoretical modeling and confirmed by experimental measurement.
Further, additional signal intensity is spread out at varying small levels in all directions, likely consisting of complex combinations of the side lobes and the annular rings, as well as imperfections due to variations in transducer sensitivity, geometric accuracy of the sensor array, and variations or imperfections of other aspects of the sodar system.
Practical phased-array sodars are usually surrounded by an open-ended enclosure. Enclosures implemented in prior designs are typically fabricated from flat panel materials. Such enclosures can perform adequately as windscreens for the transducer. However, they do little, if anything, to limit the intensity of off-axis transmission and reception for the broad range of angles which are neither in the direction of the desired beam nor nearly horizontal.
The housing is sometimes lined with open-cell foam to further reduce unwanted sounds. Sound absorbing open-cell foam sheets are commonly used in recording studios, nightclubs, and other indoor applications where echo reduction is desirable. Such foams attenuate incident sound by slowing down the vibrating air within by friction. With the right combinations of density, openness, and morphology, a broad frequency spectrum of sound energy can be absorbed. The foam surface can also contribute to echo reduction by dispersing reflected sounds. Effective dispersive surfaces can range in feature size, depending on sound wavelength, from microscopic roughness to repeating shapes several inches across.
The two most common foams used for sound absorption are open cell melamine and open cell reticulated polyurethane. Both types are very susceptible to rapid degradation from weathering, especially from ultraviolet exposure, dampness, and temperature extremes. Such plastics can be made in weatherable forms, but not types that are both weatherable and useful for sound absorption. The acoustically effective foams expose large surface areas of delicate microstructure to weathering, which can lead to rapid deterioration. Protectively coating the foam surface blocks air passage, which compromises the higher frequency sound absorption desired in sodar operation.
Sound absorption material in a sodar housing benefits sodar operation in several ways. For one, it reduces the emission of stray (off-axis) acoustic energy outside of the desired sodar pulse direction. Also, it attenuates acoustic “ringing” or “reverberation” inside the enclosure following the emitted pulse. Further, it attenuates ambient and otherwise off-axis sound energy before it arrives at the transducer array in “listen” mode.
The reduction of unwanted sound, by absorption and/or blocking, benefits the operation of sodar systems by increasing signal-to-noise ratios in a process where the signals—the reflections of emitted sounds off of atmospheric turbulence and thermal gradations—are very faint. Sound emission in unwanted near-horizontal directions may be objectionable to neighbors in some locations. Reducing such unwanted emissions as a fractional ratio of the sound emitted in the desired, upward, direction has the additional benefit that stronger sound emissions in the desired direction may be possible than would otherwise be the case without effective sound absorption and blocking. This can further improve the signal-to-noise ratio of the returned signals.
The most obvious operational orientation of the array in the wind measurement application would be horizontal, as shown with array 1000 in sodar enclosure 1010, FIG. 12, as the sound beams must be transmitted in a near-vertical pattern. In this configuration the sound mirror 1020 is omitted. This orientation has, however, the serious deficiency of being subject to precipitation or windblown debris accumulating on the array and its individual transducers. Such accumulation will at best compromise system performance; at worst it may destroy the individual transducers. A practical modification of the horizontal arrangement, also used in the prior art, is to orient the array vertically or nearly vertically as shown with array 1000a, aimed at a flat, angled, surface (sound mirror) 1020 which reflects the sound beams in the desired skyward directions. This arrangement reduces the problems presented by precipitation and falling debris by making it possible to shelter the array to a considerable degree. Even with these improvements, there is a high likelihood that precipitation and debris could be blown into the array. Testing shows that raindrops will splatter off of mirror 1020, and may enter the interior cavities of the transducers. The transducers would likely exhibit reversible performance degradation as a result of such exposure, and may still exhibit long term degradation of performance or failure from this exposure. Additionally, such an array may present an attractive site for wasps, bees, or other nesting insects, which would seriously degrade system performance.